Electrically powered automobiles are vehicles that do not depend on internal a combustion engines for propulsive power, but rather on relatively large electric traction batteries. The traction battery of an electric automobile is engaged with an electric traction motor for propelling the automobile, and the traction battery is rechargeable to permit repeated use of the traction battery.
The skilled artisan will appreciate that a traction battery must have a relatively large capacity, and must deliver a relatively large amount of power, compared to a conventional 12 volt automobile storage battery. It is further understood that because power is directly proportional to battery voltage and system current, the high power delivery requirements which must be satisfied by traction batteries necessarily mean that higher electrical voltages will be present in electric automobiles than in automobiles powered by fossil fuels, which typically require only a comparatively low power, low voltage storage battery for energizing auxiliary loads when the internal combustion engine is not operating.
Hybrid electric vehicles (BEVs) combine the internal combustion engine of a conventional vehicle with the battery and electric motor of an electric vehicle. This results in an increase in fuel economy over conventional vehicles. This combination also offers extended range and rapid refueling that users expect from a conventional vehicle, with a significant portion of the energy and environmental benefits of an electric vehicle. The practical benefits of HEVs include improved fuel economy and lower emissions compared to conventional vehicles. The inherent flexibility of HEVs also permits their use in a wide range of applications, from personal transportation to commercial hauling.
Because electric or hybrid electric vehicles require little or no combustion of fossil fuels, such vehicles produce little or no environmentally harmful emissions, in contrast to fossil fuel powered vehicles. Such vehicles are become increasingly attractive alternatives to fossil fuel powered cars. However, because of the high voltage requirements of its traction battery an electric or hybrid electric vehicle raises significant electrical safety concerns.
For example, unwanted electric current flow outside of the intended electric circuit flow (i.e. ground fault conditions) may cause significant damage to electronic components within a system (such as an HEV propulsion system), thereby disabling or even destroying the electronic equipment. In addition, such ground fault conditions may result in an electric shock, which can have graver consequences when the shock is caused by contact with a high voltage traction battery system, as compared to a conventional, relatively low voltage automotive storage battery system. To reduce the likelihood of such shock, many traction battery systems are not grounded to the automobile chassis, in contrast to conventional automotive storage battery systems. Instead, many traction battery systems have a closed loop return path, so that the negative power conductor of the system (i.e., the electrical current return path) is isolated from the chassis of the electric or hybrid electric vehicle.
While such isolated systems may minimize the likelihood of a significant electric shock to a person in the event of a short circuit or low impedance connection between a phase lead of a motor and the chassis (i.e a ground fault), certain electronic components typically in electrical communication with the positive and negative power conductors or rails that supply power to drive the motor, are subject to damage resulting from extreme voltage or current swings occurring on the positive and negative rails. Further, while substantial work has been performed on detecting DC ground faults in high voltage DC systems, such DC ground fault detection schemes fail to detect ground faults on AC outputs of the DC system such as motors, generators or other loads. A ground fault detection system for sensing an AC ground fault condition associated with a given load such as a motor and operable to interrupt power thereto is desired.